Radio receiving set



Dec. 25, 1934. s. w. SEELEY ET AL RADIO RECEIVING SET Filed Oct. 20, 1932 AMPLTFIER Aumo RMPUFIER DETECTOR osu Lnmk DETECTOR Patented Dec. 25, 1934 F TE NT OFFICE 1.985395 RADIO RECEIVING "s 'r' Stuart W. Seeley andGarrard-Mountjoy, Jackson, Y

I Mich., assignors, .by mesne assignmentafto' Radio Corporation oIAmerica. New Y. ork, N. 1L,

V a corporation of pclaware Application October '20. 1.032, Serial? No. 638,752 3 s ciai sihtolfizsozoy This invention relates to radio receivingsets andis particularly useful in receiving 'setsof thesuperheterodyne type where an oscillator beats with the desired signal to produce a-super-audible intermediate frequency.

In superheterodyne setsthe oscillator combines with two frequencies to produce'the 'sameintermediate frequency. One of these isabove the oscillator frequency and the other one below it. For example, a" superheterodyne oscillator hav-'- ing a frequency of 1175 k. (3.Wi11 produce an intermediate frequency of 175 k. c. with a signal of 1000 k. c. or with another signal of 1350 k. c. -In'superheterodyne receiving sets a number ofjcircuits tuned to the desired signalfrequency precede the frequency changer or first detector andt-hus amplify the desired signal withoutappreciably amplifying the other undesiredfrequency which is usually called theimage of the desired signal," However, unless an undue amount of radio frequency tuning is employed. there still remains anjobjectionable amount of image frequen ya interferewithfthe signal. "It'is'the object of our invention'to use a 1mm "amount of radio frequency tuning ahead time oscillator to lower the cost'and to eliminate thelimage frequency by balance circuits which are" oft relatively low cost compared to the .usual tuning stages. j The manner in which the'balance circuits are employed will now, be-described, reference beinghad tothedrawing in which: q "Fig.1 is anillustration of the circuits wh ch the invention is used, the parts {not directly involved in the invention being shownconvention ally.

a simpler form and disconnectedjfrom the rest of the receiving set.

iRefe'rring to Fig. 1,..the antenna lis connectaiid coupling condenser 2 and ground ,3 through a timed couple. consisting of inductance i l capacity ,5, the latter being Variable to permit tuning for various signals. -One stage of tuning is allthat is employed in the illustrated circuit andthe "resonant voltage-of the. tunedcouple is applied directly to the gridfi of a-vacuum tube 6,: .Thisvacuum, tube is shown as a three-elementtube forsimplification but of courseascreen grid .tube:.or aflpentode .(five elementi tubehor other suitable tube may -be:used'." The filament 7 in practice u'sually 'is fed from a power supply but for-simpliciti "it-is illustrated as connected to a simple A source 8, the filament be- .Fig. 2 is an illustration of the balance circuits ing connected to ground 3 through asmall condenser 9 and resistance 10. q

' The plate of the tube 11-is connected to the first detectorillustrated conventionally and to the positiveterminal ofthe Bsupply preferably through-a radio frequency choke 12. The negative terminal of the 'B supply is connected to ground but to reduce the resistance we preferably employ aresistor 13 in shunt to the series circuit containing condenser 9 and resistance 10. Iii-practice; it is usual to have one power supply to furnishthe filament and plate voltages, but forsimplicity, thecomplete power apparatus has not been shown. 0

In general, the desired signal received on the antenna 1- is increased in voltage by the tuned circuit 4, 5, and amplified byfthe vacuum tube 6. This amplified signal is then introduced into the detector circuit 14 where it combines with the output of oscillator 15 and is detected to produce the super-audible intermediate "frequency; ,This intermediate frequency is then amplified at 16 and detected again at 17 to produce an audio frequency. This audio frequency is amplified at 1'8and heard in the phones 19 which in practice usually would be s'omeform of loud speaker. While the signal is being received the image frequency is also impressed on the grid 6 of the radioffrequencyamplifier if a station of that frequency is operating at the time. While this. image frequency doesnot have its voltage materially increased by-the tuned couple 4,5, it nevertheless is of such value as to produce an objectionable-interfering signal in the indicating device'19. The mannerin which this image frequency is balanced out willbe best understood by referring .t'otthe circuits inFig. 2. This figure illustrates the six arm iWheatstone bridge circuit. Imthisrbridge the varmE consists .of the antennaground circuit and the conjugate. arm F consists of the vacuum tube amplifier and its associated first detector and .heterodyne oscillator, the lat-' terbeing omitted in Fig. .2 for simplicity. The arm A consistsof .the capacity 2 while its conjugate arm Dfconsists ofthe capacity .9 and the resistances 10'and 13. The arm B consists of the inductance 4 and capacity 5 made resonant to the desiredjsignal and its cdnjugate arm, C consistsof a condenser 20 and inductance 21 having a resonantfpoint above theima ge of the signal-'to which'the loop 4, 5 iscapable ofbeing tunedf' A l -Sincethe grid-'6 is connected to the point 22 of this bridge and the filament is connected to the point 23 thereof the desired signal is im- 2 L' r I pressed directly between the grid 6 and the filament '7 because the tuned circuit 4, 5, unbalances the circuit for the desired signal. By properly proportioning the condenser 20, coil 21, condenser 9 and resistances 10 and 13, the bridge may be substantially balanced for the image frequency throughout the broadcast band so that the points 22 and23 are at substantially the same potential in respect to the image.

The invention is not limited to any particular proportiom'ng of parts but, by way of example, we will say that good results have beenobtained by making the coupling condenser 2 of .36 micromicrofarads capacity, the'coil 4 of 220 microhenrys, the variable capacity '5 with a maximum capacity of 400 micro-microfarads, the condenser 20 of 43.95 micro-microfarads, the coil 21 of 289 microhenrys, condenser 91 of" 500 micro-microfarads, and resistances 10 and 13 of 8 ohms and 8,500 ohms respectively. Assuming that the desired signal'for a certain setting of the tuning arrangement is 1000 k. c. and assuming k. c. as the intermediate frequency, the oscillator would have a frequency of 1175 k. c. and the image frequency would be 1350 k. c. With these assumptions the apparent or effective capacity of the B arm would be about 35. micro-microfarads for the image frequency because above the resonantfrequency the B arm exhibits a capacitativereaction. When the tuning unit is adjusted to 550 k. c. the image producing frequency would be 900 k. c. and the B arm would have an effective capacity of about 250 micro-microfarads for that frequency.

'Io produce substantially no potential drop between the points 22, 23 of the bridge circuit the arms must be proportionalso thatthe product of the impedances of the-conjugate arms A and D equals the product of the impedances of arms BandC. I

Since theimage frequency is always above the signal frequency for theassumed case, theB arm appears as a decreasing effective capacity as the frequency increases. In the C arm, theinductive reactance of-inductor 21 subtracts from the capacitative reactance of capacity 21 such that the arm appears as a larger eifective capacity as the frequency increases. Hence, as the. couple in loop 4, 5, tunesto higher signals in the reception band the-balance for the corresponding image frequencies will substantially remain due to the compensating efiect in the B and C arms. V ,The inverse changes ,in apparent capacity of the arms B and C do not affect the reception of the desired signal as the tuned arm Bvgreatly ofisets any small change in capacity in the C arm. The balancing of image frequency is hindered by a considerable resistance component in the B arm at the higher frequencies and to compensate for this the small resistor- 10 is placed in series with the capacity 9. Again this has no effect on the reception of the desired signal. In this balance circuit the capacity 9 is quite small and it is advisable to by-pass the radio frequency. currents in the plate circuit directly tothe cathode through condenser 24. v In receiving sets that we have designed employ-- ing this invention .we have reducedthe image frequency to a point where it is entirely inaudible overthe entire broadcast band, which with only one tuned'circuit ahead of'the first detector productes a. very satisfactory receiver at-quite low cos 1 of conjugate arms effective capacities that vary substantially inversely with frequency changes within the band defined by the signal and image frequencies.

2. In asuperheterodyne receiving set, a bridge circuit consisting of a signal and image frequency source in one arm, means connected to the conjugate of said arm for producing beats with the signaland image frequencies, means for producsubstantially constant effective capacity in each of another pair of conjugate arms, a loop in one arm of the third pair of conjugate arms tuned substantially tothe signal frequency and a condenser and inductance in the other arm of the third pair of conjugate arms tuned to a frequency other than the signal frequency.

3. In a superheterodyne receiving set, a bridge circuit consisting of a signal and image frequency source in one arm, means connected to the conjugate of said arm for producing beats with the signal, means for producing substantially constant effective capacity in each of another pair of conjugate arms, a loop in one arm of the third pair of conjugate arms substantially tuned to the signal frequency and acondenser and inductance-in series in the other arm of the third pair of conjugate arms tuned to a frequency beyond the image frequency.

4. In a superheterodyne receiving set, a bridge circuit consisting of a signal and image frequency source in one arm, means connected to the, conjugate of said'arr'n for producing beats with the signal, condensers of substantially constant capacity in the arms of a second pair of conjugate arms, an inductance and condenser in parallel in one arm of .thethird pair of conjugate arms, and a condenser and inductance in series in the other arm of said third pair of conjugate arms.

5. In a superheterodyne receiving set, a bridge circuit consisting of a signal and image frequency source in one arm, means connected to the conjugate of said arm for producing beats with the signal, condensers of substantially constant capacity in the arms of a second pair of conjugate arms, an inductance and condenser in parallel in one arm of the third pair of conjugate arms and substantially tuned to the signal, and a condenser and inductance in series in the other arm of said third pair of conjugate arms and tuned above the image frequency. g 6. In a superheterodyne receiving set, a. bridge circuit consisting of a signal and an image frequency source in one arm, a beating frequency source connected to the conjugate of said arm, impedance elements in another pair of conjugate arms whose ratio of impedances is substantially independent of frequency, impedance elements in athird pair of conjugate arms whose ratio of 'impedances inherently varies with frequency, and means to adjust one of the last mentioned impedance elements to balance the bridge for the imagefrequency source and unbalance it for the Signal frequency source.

- I STUART W. SEELEY.

GARRARD MOUNTJOY. 

